Parasitic protozoa are responsible for a wide variety of infections in man and animals, many of which are life threatening to the host. For example, members of the phylum Apicomplexa (formerly called Sporozoa) comprise a large and diverse group of pathogenic protozoa that are intracellular parasites which vary tremendously in virulence depending upon the species with which the host is infected. Some species cause diseases that result in mild symptoms that might go unnoticed (i.e., mild diarrhea) and eventually disappear, while other species cause highly virulent infections that are rapidly fatal. In animal husbandry, many of these diseases can cause considerable economic loss to the owners, as many such diseases can cause effects to the animals which may not be apparent until the animal is prepared for market. For example, poultry are generally marketed within weeks after hatching and this short time period does not permit the animals sufficient time to recover from a chronic infection prior to market if not treated in time.
A protozoal infection of great economic importance in the food industry is coccidiosis, a widespread disease of domesticated animals caused by protozoa of the genus Eimeria. Some of the most significant of Eimeria species are those in poultry, namely E. tenella, E. acervulina, E. necatrix, E. brunetti, E. praecox, E. mitis and E. maxima. The disease is responsible for high levels of morbidity and mortality and can result in extreme economic losses. Accordingly, there exists a continued need to identify new and effective anti-protozoal drugs.
While coccidiosis occurs in several domesticated and wild animal species, such as cattle, sheep, pigs and rabbits, the disease is especially important in poultry, in particular chickens. See Williams, R. B., A compartmentalized model for the estimation of the cost of coccidiosis to the world's poultry production industry, Int. J. Parasitology, 29: 1209-1229 (1999).
The disease is caused by the replication within the intestine of the asexual and/or sexual stages of seven species of Eimeria (usually several species occur concurrently) and although clinical disease in intensively reared poultry is now relatively uncommon, sub clinical infections are the norm. The poultry disease is generally spread by the birds picking up the infectious organism in droppings on contaminated litter or ground or by way of food or drinking water. The disease is manifested by hemorrhage, accumulation of blood in the ceca, passage of blood to the droppings, weakness and digestive disturbances. The disease often terminates in the death of the animal but the fowl which survive severe infections have their market value substantially reduced as a result of the infection. Of the seven species of Eimeria, the most pathogenic species are E. necatrix and E. tenella, followed by E. maxima. The least pathogenic species is E. praecox. 
The total cost of coccidial infections in the US runs in the hundreds of millions of dollars. In the UK alone, the total cost, corresponding to about 780 million broilers, has been estimated to be at least £42 m per annum, of which 74% is due to sub-clinical effects on weight gain and feed conversion and 24% is the cost of prophylaxis and therapy of commercial birds. See Williams, supra;
Presently, poultry flocks are protected against coccidiosis by either immunization or the use of anti-coccidial chemotherapeutic agents with the most commonly utilized method for controlling coccidial infections being the use of anti-coccidial agents. The most successful anticoccidials have been the polyether ionophores, a class of antibiotics of complex structure, which continues to be the industry standard since its introduction nearly thirty years ago. While many such ionophores exhibit anti-coccidial activity, the relative degree of activity varies from one agent to another. As well, the use of ionophores for controlling coccidiosis in animals requires that the agent be continuously administered to the animal typically by being mixed with the feed used for the entire growth period. Reports of resistance development due to the extended and constant chemotherapeutic pressure exerted by this class of compounds are not uncommon. See Stephen, B. et al., Vet. Parasitol., 69: 19-29 (1997) and Daugschies et al., Vet. Parasitol., 76: 163-171(1998). As such, there is a dire need to develop antiprotozoal compounds that are not attended by the above referenced drawbacks.
Probably the most well-studied acomplexicans are species of Plasmodium, an obligate intracellular parasite, which is the causative agent for malaria. Malaria remains a significant health threat to humans despite massive international attempts to eradicate the disease. There are four species of Plasmodium which infect humans. P. falciparum, P. vivax, P. malariae, and P. ovale. 
Alone among the malaria species which infect humans, Plasmodium falciparum causes the erythrocytes, which it invades to sequester in the deep vascular beds of various tissues. Sequestration allows the parasite to develop in a microenvironment of low oxygen tension and to evade splenic immune surveillance (Langreth and Peterson (1985) Infect. Immun. 47: 760-766). P. falciparum is the most virulent species of malaria since it can cause the death of the host as a result of: cerebral malaria, pulmonary or renal failure.
More than ten years ago an urgent need for drugs against malaria was identified. The antibiotics currently in use for the treatment and prophylaxis of malaria (including the tetracyclines and clindamycin) have little action on pre-erythrocytic stages and act slowly on blood stages, but are used for treatment of drug resistant strains because of their safety rather than their efficacy. Furthermore, the rapid spread of resistance to chloroquine has heightened the need for ready availability of relatively low cost prophylactic and therapeutic anti-malarial drugs. Owing to the devastating consequences of the disease, and the potential for therapeutic intervention, researchers have long sought to identify the parasite protein(s) responsible for the devastating effects attending Plasmodium falciparum infection.
Although discovered in 1963, the importance of the cyclic nucleotide cofactor cGMP in many cellular and physiological processes was first described in the late 1970s (Lincoln and Cornwell, 1993; Baumner and Nawrath, 1995; Moro et al., 1996; Vaandrager and de Jonge, 1996). cGMP is generated by guanylate cyclase (GC) and degraded by specific cyclic nucleotide phosphodiesterases (PDE). It exerts its intracellular effects by interacting with a group of intracellular proteins known as intracellular cGMP receptor proteins (Lincoln and Cornwell, 1993), which include cGMP-dependent protein kinases (PKGs), cGMP-dependent phosphodiesterases (PDEs), and cGMP-gated ion channels.
It is believed that the binding and activation of cGMP-dependent protein kinase (PKG) is responsible for most of the intracellular actions of cGMP. PKGs are known to control many cellular processes in higher animals. Although expressed in many cell types, these serine/threonine protein kinases are found at high levels in mammalian lung, cerebellum, platelets, and SMCs (Francis and Corbin, 1994). cGMP dependent protein kinases (PKG) catalyze the phosphorylation of specific protein substrates. In the absence of cGMP the activity of these enzymes is very low.
The canonical phosphorylation site for PKG substrates (Arg-Arg-X-Ser) is the same described for cAMP-dependent protein kinases (PKA, Vaandrager and de Jonge, 1996). In mammalian cells there are two types of PKG, a soluble (PKG1) and a membrane bound form (PKG2). (Vaandrager and de Jonge, 1996). Multiple splice variants of the soluble protein have been identified. Type I -PKG1 has two isoforms (type I-α and type I-β), while type If is less common and expressed only in intestinal epithelial cells (Lincoln and Cornwell, 1993), kidney, and brain (Vaandrager and de Jonge, 1996).
The present invention overcomes previous shortcomings in developing effective treatments of various types of parasitic infections by providing the nucleotide sequence of the gene encoding Eimeria maxima PKG and Plasmodium falciparum PKG respectively, the novel protein kinases encoded by each of the nucleic acid molecules, and the use of the proteins as chemotherapeutic target(s). Therapeutics identified via use of the novel nucleic acids as well as the herein disclosed assays are likewise contemplated by the present invention.